Tropical Storm NEPARTAK Advisory Wed Jul 28

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 11W (NEPARTAK) WARNING
NR 016//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 35.8N 142.3E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 130 NM EAST-NORTHEAST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
11W (NEPARTAK) REMAINS ASSESSED AS A SUBTROPICAL STORM. ANIMATED
MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS SHALLOW BANDING
WRAPPING INTO A VERY WELL-DEFINED AND FULLY EXPOSED LOW LEVEL
CIRCULATION CENTER (LLCC). ANIMATED WATER VAPOR IMAGERY, UPPER-AIR
OBSERVATIONS AND MODEL ANALYSIS INDICATE THAT THE LLCC ASSOCIATED
WITH 11W IS CURRENTLY UNDER A COLD UPPER-LEVEL LOW WITH DRY AIR
ADVECTING INTO THE CIRCULATION, PARTICULARLY ACROSS THE SOUTHERN
HEMISPHERE. THE CIRA THERMAL CROSS-SECTION ANALYSIS VALID AT
270000Z CONFIRMS THE WARM OVER COLD ANOMALY STRUCTURE ASSOCIATED
WITH SUBTROPICAL SYSTEMS. SCATTERED AND DISORGANIZED CONVECTION IS
OCCURRING IN THE NORTHERN SECTORS OF THE CIRCULATION UNDER THE COLD
POOL ALOFT, WHICH IS SLIGHTLY OFFSET FROM THE UPPER-LEVEL LOW
CENTER. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED
ON THE EXPOSED LLCC, SUPPORTED BY ANIMATED RADAR DATA AND RJTD
RADAR FIX POSITIONS. THE INITIAL INTENSITY IS ASSESSED WITH
MODERATE CONFIDENCE, HEDGED HIGHER THAN THE SUBTROPICAL DVORAK
INTENSITY ESTIMATES OF T2.5 FROM PGTW, IN LIGHT OF THE HIGHER
SATCON VALUES AND PERSISTENCE WITH EARLIER SCATTEROMETER DATA.
ADDITIONALLY, SURFACE PRESSURES FROM CHOSHI, JAPAN BOTTOMED OUT AT
984 MB, WHICH SUPPORTS A CENTRAL PRESSURE IN THE 988 MB RANGE,
EQUIVALENT TO APPROXIMATELY 45 KTS.
INITIAL WIND RADII BASIS: PERSISTENCE
CURRENT STEERING MECHANISM: MID-LEVEL, NORTH TO SOUTH ORIENTED
SUBTROPICAL RIDGE (STR) CENTERED TO THE EAST.
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   CIMSS SATCON: 51 KTS AT 262139Z
   CIMSS ADT: 43 KTS AT 270600Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 0-5 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: WEAK WESTWARD
   OTHER FACTORS: WHILE VWS IS LOW AND SSTS ARE HIGH, THE OVERALL
ENVIRONMENT IS UNFAVORABLE FOR TROPICAL CYCLONE DEVELOPMENT DUE TO
THE PRESENCE OF THE UPPER-LEVEL LOW OVER THE LLCC.
ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: MEDIUM
3. FORECAST REASONING.
SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.
FORECAST DISCUSSION: AFTER A SHORT PERIOD OF UNEXPECTED
WEST-SOUTHWEST MOVEMENT EARLIER IN THE DAY, 11W IS NOW FIRMLY
ENTRENCHED ON A NORTHWESTWARD TRACK. THE SYSTEM IS EXPECTED TO MAKE
LANDFALL IN THE VICINITY OF SENDAI, JAPAN WITHIN THE NEXT 12 TO 15
HOURS, THEN TRANSIT QUICKLY ACROSS THE NORTHERN PORTION OF HONSHU
BEFORE REEMERGING INTO THE SEA OF JAPAN NEAR TAU 24. THE SYSTEM
WILL THEREAFTER TURN ONTO A MORE WESTWARD TRACK WHILE SLOWING
RAPIDLY AS IT MOVES INTO A WEAK STEERING ENVIRONMENT. THE SYSTEM IS
FORECAST TO WEAKEN SLIGHTLY PRIOR TO LANDFALL, BUT IS STILL
EXPECTED TO BE A SUBTROPICAL STORM AT LANDFALL. CONTINUED WEAKENING
IS FORECAST AS IT CROSSES THE JAPANESE ALPS, REEMERGING OVER WATER
AS A 30 KT SUBTROPICAL DEPRESSION. AS THE SYSTEM CONTINUES TO MOVE
WEST THROUGH TAU 72, THE UPPER-LEVEL LOW WHICH HAS BEEN SHIELDING
THE SYSTEM FROM HIGH VWS WILL BEGIN TO SHIFT BACK EASTWARD,
ALLOWING THE CIRCULATION ASSOCIATED WITH 11W TO MOVE UNDER AN AREA
OF VERY HIGH VWS ASSOCIATED WITH A 200 MB JET MAX, LEADING TO RAPID
WEAKENING AND DISSIPATION NO LATER THAN TAU 72, AND POTENTIALLY AS
EARLY AS TAU 48.
MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN GOOD
AGREEMENT THROUGH TAU 36 WITH A SPREAD OF 95 NM BETWEEN THE EXTREME
OUTLIERS HWRF AND JGSM. HOWEVER THE JTWC FORECAST TRACK LIES CLOSE
TO BUT SLIGHTLY SLOWER THAN THE MULTI-MODEL CONSENSUS MEAN AND
AMONGST THE TIGHT GROUPING OF THE MAJORITY OF THE CONSENSUS
MEMBERS. AFTER TAU 36, THE MODELS SHOW SIGNIFICANT DIVERGENCE IN
SOLUTIONS, WITH THE JGSM AND ECMWF ENSEMBLES SHOWING A TURN TO THE
NORTH-NORTHEAST, WHILE THE HWRF AND GALWEM INDICATE THE WEAKENING
CIRCULATION TURNING SHARPLY SOUTHWARD AFTER TAU 36. THE REMAINDER
OF THE CONSENSUS MEMBERS SUPPORT THE JTWC FORECAST TRACK TO THE
WEST THROUGH TAU 72, THOUGH WITH ONLY MEDIUM CONFIDENCE. THE JTWC
INTENSITY FORECAST LIES CLOSE TO, BUT SLIGHTLY LOWER, THAN THE
INTENSITY CONSENSUS MEAN THROUGH THE DURATION OF THE FORECAST
PERIOD, WITH MEDIUM CONFIDENCE DUE TO THE UNCERTAINTIES ASSOCIATED
WITH THE INFLUENCE OF TERRAIN INTERACTION AND THE ULTIMATE STRENGTH
OF THE LLCC THAT EMERGES INTO THE SEA OF JAPAN.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   INTENSITY 0 - 72 HR: MEDIUM//
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